Get Previous Value in Column

I'm not sure if you want this and what is your source, but if you only want to shift 3 rows, you can do that this way.

let
    fnShift = (tbl as table, col as text, shift as nullable number, optional newColName as text, optional _type as type) as table =>
        //v 3. parametri zadaj zaporne cislo ak chces posunut riadky hore, kladne ak dole, 4. je nepovinny (novy nazov stlpca), 5. je nepovinny typ
        let
            a = Table.Column(tbl, col),
            b = if shift = 0 or shift = null then a else if shift > 0
                then List.Repeat({null}, shift) & List.RemoveLastN(a, shift)
                else List.RemoveFirstN(a, shift * -1) & List.Repeat({null}, shift * -1),    
            c = Table.FromColumns(Table.ToColumns(tbl) & {b}, Table.ColumnNames(tbl) &
                ( if newColName <> null then {newColName} else
                    if shift = 0 then {col & "_Duplicate"} else
                    if shift > 0 then {col & "_PrevValue"} 
                    else              {col & "_NextValue"} )),
            d = Table.TransformColumnTypes(c, {List.Last(Table.ColumnNames(c)), if _type <> null then _type else type any})
        in
            d,
            
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjDSNzLUN7RUitVB51jqAxFpHENDfQMTwhwDoCWm+kYGVODEAgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date = _t]),
    GroupedRows = Table.Group(Source, {"Date"}, {{"All", each _, type table}}),
    Ad_DatePrevValue = fnShift(GroupedRows, "Date", 1),
    Ad_Data = Table.AddColumn(Ad_DatePrevValue, "Data", each Table.AddColumn([All], "Start Date", (x)=> [Date_PrevValue]), type table),
    Data = Table.Combine(Ad_Data[Data])
in
    Data